In recent years, one type of such a turbo refrigerator has been proposed that is configured to use water as a refrigerant instead of a greenhouse effect gas, such as chlorofluorocarbon, as an environmental measure. In such a turbo refrigerator, the water having a higher boiling point than the chlorofluorocarbon is evaporated under low pressure, so that the refrigerant decreases in density and increases in volume flow rate. Therefore, the turbo compressor tends to increase in size. In contrast, heat exchangers, such as a condenser and an evaporator, do not increase in size as much as the turbo compressor does since the water has better thermal conductivity than the chlorofluorocarbon.
To be specific, although the devices increase in size, the turbo compressor, the condenser, and the evaporator do not increase in size at an equal rate, but only the turbo compressor increases in size as compared to the other components. Therefore, in a case where a typical structure of a chlorofluorocarbon turbo refrigerator in which a turbo compressor and a heat exchanger are formed as separate components and are connected to each other via a pipe is applied to a water refrigerant turbo refrigerator, only the turbo compressor increases in size, and a large dead space remains around a centrifugal impeller.
In a case where the pipe and the like are reduced in size as much as possible in order to suppress increases in sizes of the devices as much as possible, the flow velocity of the refrigerant tends to increase, and this increases the pressure loss. Thus, the performance of the turbo refrigerator deteriorates.
To solve these problems, proposed is a turbo refrigerator in which impellers of a two-stage centrifugal turbo compressor are arranged back-to-back (Japanese Patent No. 4191477). Instead of collecting by a scroll a refrigerant radially flowing out and then introducing the refrigerant to a pipe extending to a condenser, in the turbo refrigerator, a plurality of diffuser ducts are provided for each of the first-stage and second-stage impellers, and the first-stage diffuser ducts and the second-stage diffuser ducts are arranged alternately in a circumferential direction.